A fuel tank has a fuel containing region and an ullage region. The ullage region contains a gas that comprises a fuel/air mixture. In order for the mixture to burn it is necessary to have certain proportions or concentrations of fuel to air (or oxygen). Those concentrations that support burning or combustion fall within a flammability range.
When the fuel particles are so widely separated that those set on fire by ignition will not set fire to others that are nearer, the mixture is called too lean for combustion and will not burn. When the particles are so close together that they exclude the oxygen necessary for combustion, the mixture is called too rich and will not burn. A flammability or combustion supporting range for a fuel/air mixture is defined by a lower limit concentration (or percent of fuel by volume) below which the mixture is incombustible and an upper limit concentration above which the mixture is incombustible.
The lower limit represents the minimum concentration that will ignite and burn, while the upper limit represents the maximum concentration that supports combustion. The generally accepted flammability range by volume for jet A turbine fuels is from about 0.74 to 5.32%. Jet B turbine fuels have a flammability range from about 1.16 to 7.63%. Most types of gasoline have a flammability range from about 1.4 to 7.6%.
There are a number of circumstances that can cause drastic changes in concentration that result in fuel/air mixture in the ullage region of a fuel tank to enter the hazardous flammability range such that a hot point or electrostatic spark could ignite the mixture thereby leading to a tank fire or explosion. These circumstances include a fuel consumption that removes fuel from the tank, a descent in altitude, entry of air into the tank or a fast reduction in tank ullage pressure after take off when the aircraft reaches high altitude in a short time, with the fuel still at the ambient ground temperature on a hot day (for example, 98.degree. F.).
Prior art systems, exemplified by U.S. Pat. Nos. 3,628,758, 3,229,446 and 3,788,039, have used a fuel ignition arrester system which injects an inert gas, such as nitrogen, in sufficient amounts into the tank ullage to keep the fuel/air concentration below the lower limit of the flammability range. This system requires nitrogen storage or generation equipment that is expensive to install on an aircraft, is heavy and also is hazardous.
The present invention is generally directed to providing novel and improved systems and methods for flammability detection and fuel ignition arrest. The novel system is inexpensive, light weight and less hazardous than the prior art inert gas systems. The unique fuel ignition arrester system and method of the present invention maintains the fuel air mixture in the tank ullage above the flammability range.